1. Field of the Invention
The present invention relates to a method for determining a constant in a relational expression concerning the end-tidal air velocity constant and the arterial blood velocity constant, and a xenon CT apparatus. In particular, the present invention relates to a method for determining a constant in a relational expression concerning the end-tidal air velocity constant and the arterial blood velocity constant, and a xenon CT apparatus which make it possible to correctly measure the cerebral blood flow rate by utilizing, for example, a xenon gas supply unit and an X-ray CT unit.
2. Description of the Related Art
A method is known, for example, in which a tomographic image of a head of a patient as a specimen is obtained by using an X-ray CT apparatus, while the patient is allowed to inhale a mixed gas composed of xenon gas and oxygen gas fed from a gas inhalation unit for a certain period of time by the aid of a breathing mask, and then the patient is allowed to breathe ordinary air, wherein the tomographic image is analyzed to measure the blood flow in the head of the patient.
That is, according to this measuring method, the mixed gas is absorbed into the pulmonary vein from the lungs of the patient, it passes through the heart, and it flows as the arterial blood flow into the tissue of the head. The mixed gas passes through the head tissue, it returns to the heart via the venous blood flow, and it is returned to the pulmonary artery via the heart. During this process, the time-dependent change of the xenon gas concentration in the tissue of the head is observed by using the X-ray CT apparatus, and it is compared with the time-dependent change of the xenon gas concentration of a head in which the tissue is normal. Thus, it is possible to diagnose the head of the patient.
In order to obtain the cerebral blood flow rate by using the measuring method described above, it is necessary to obtain the xenon gas concentration in the artery together with the xenon gas concentration in the cerebral tissue. Recently, the xenon gas concentration in the end-tidal air, which can be detected by a noninvasive method, is substitutively used as the xenon gas concentration in the artery.
The inventors of this application have found out and revealed the correlation between the xenon gas concentration in the artery and the xenon gas concentration in the end-tidal air {see xe2x80x9cThe Effect of Xenon Inhalation Speed on Cerebral Blood Flow Obtained Using the End-Tidal Method in Xenon-Enhanced CTxe2x80x9d, Shigeru Sase, Journal of Computer Assisted Tomography, 22 (5): 786-791, 1988}.
The correlation is as follows. That is, the velocity constant of the xenon gas concentration in the artery is expressed by a linear exponential function of the velocity constant of the xenon gas concentration in the end-tidal air by using a conversion constant.
This invention has been made taking the knowledge as described above into consideration, an object of which is to provide a method for determining a constant in a relational expression concerning the end-tidal air velocity constant and the arterial blood velocity constant, and a xenon CT apparatus which make it possible to correctly determine the cerebral blood flow rate by using the xenon gas concentration in the end-tidal air by utilizing the correlation between the xenon gas concentration of the blood flow in the artery and the xenon gas concentration in the end-tidal air.
According to the present invention, there is provided a method for determining a constant xcex3 in a relational expression:
Ka=xcex3xc3x97(1xe2x88x92exp(xe2x88x92Ke/xcex3))
wherein Ke represents an end-tidal air velocity constant and Ka represents an arterial blood velocity constant in a xenon CT examination, the method comprising a step A of setting a region of interest on a xenon CT image; and a step B of determining the constant xcex3 with which a xenon distribution coefficient xcex most closely approaches a predetermined target value in the preset region of interest. As described above, it is possible to obtain a correct value of the constant xcex3 by determining the constant xcex3 by using the xenon distribution coefficient xcex as an index.
In the step B, the distribution coefficient xcex is calculated by varying the constant xcex3 within a desired range from 0.24 to 7.7 to determine the constant xcex3 with which the distribution coefficient xcex most closely approaches the target value. As described above, the range of the constant xcex3 to be considered is limited, and thus it is possible to shorten the processing period of time required to determine the constant xcex3.
In this case, it is also preferable that the desired range is a range from 0.3 to 2.5.
In the step B, the constant xcex3, with which a value of the distribution coefficient xcex most closely approaches the target value, is determined for each of predetermined picture elements included in the region of interest, and obtained values of the constant xcex3 are averaged to estimate an objective value of the constant xcex3. Accordingly, it is possible to determine the constant xcex3 more correctly.
Further, in the step A, the region of interest is set to a region including cerebral white matter, and in the step B, the target value is determined depending on a hematocrit value. Accordingly, it is possible to correctly determine the constant xcex3 in order to determine the cerebral blood flow rate.
According to the present invention, there is provided a xenon CT apparatus comprising a gas supply unit for supplying xenon gas to a specimen; a concentration-measuring unit for measuring a xenon gas concentration (hereinafter referred to as xe2x80x9cexpiration gas xenon gas concentrationxe2x80x9d) in end-tidal air of the specimen; a main X-ray CT apparatus body for obtaining CT image data of an examination site in order to obtain a xenon gas concentration (hereinafter referred to as xe2x80x9cexamination site xenon gas concentrationxe2x80x9d) of the examination site of the specimen; and a data processing unit for determining the examination site xenon gas concentration on the basis of the CT image data, and determining a blood flow rate of the examination site on the basis of the examination site xenon gas concentration and the expiration gas xenon gas concentration; wherein the data processing unit determines a xenon gas distribution coefficient xcex between the examination site and blood of the specimen on the basis of a conversion constant xcex3 for converting a velocity constant (hereinafter referred to as xe2x80x9cexpiration gas velocity constantxe2x80x9d) of the expiration gas xenon gas concentration into a velocity constant (hereinafter referred to as xe2x80x9carterial blood velocity constantxe2x80x9d) of a xenon gas concentration (hereinafter referred to as xe2x80x9carterial xenon gas concentrationxe2x80x9d) of blood flow in artery, and it establishes, as a true value, the conversion constant xcex3 with which the distribution coefficient xcex most closely approaches a predetermined target value. As described above, it is possible to obtain a correct value of the conversion constant xcex3 by determining the conversion constant xcex3 by using the distribution coefficient xcex as an index.
In this arrangement, the data processing unit includes a conversion constant-setting means for determining the conversion constant xcex3; and the conversion constant-setting means has an assumed value-setting means for setting an assumed value of the conversion constant xcex3 and varying the assumed value.
The conversion constant-setting means includes a temporary velocity constant-setting means for determining a temporary calculated value of the arterial blood velocity constant from the expiration gas velocity constant on the basis of the assumed value of the conversion constant xcex3; a temporary distribution coefficient-calculating means for determining a temporary calculated value of the distribution coefficient xcex from the temporary calculated value of the arterial blood velocity constant and the examination site xenon gas concentration; and a conversion constant-extracting means for extracting, as an extracted assumed value to be established as the true value, an assumed value of the conversion constant xcex3 corresponding to one which most closely approaches the target value, of respective temporary calculated values of the distribution coefficient xcex obtained on the basis of respective assumed values of the conversion constant xcex3.
The conversion constant-setting means includes a filtering means for determining, as a filtered value to be established as the true value, one in which the temporary calculated value of the distribution coefficient xcex corresponding to the extracted assumed value is included in a predetermined filtration range, of the extracted assumed values.
The conversion constant-setting means includes an ROI data-extracting means for extracting examination site xenon gas concentrations respectively obtained on the basis of data corresponding to a plurality of predetermined picture elements, of data of respective picture elements included in the CT image data; and an average value-calculating means for determining, as the true value of the conversion constant xcex3, an average values of the filtered values obtained on the basis of the extracted examination site xenon gas concentrations respectively.
Accordingly, it is possible to determine the conversion constant xcex3 more correctly.
The assumed value-setting means varies the assumed value of the conversion constant xcex3 within a desired range from 0.24 to 7.7. As described above, the range of the assumed value to be considered is limited, and thus it is possible to shorten the processing period of time required to determine the conversion constant xcex3.
In this arrangement, it is also preferable that the desired range is a range from 0.3 to 2.5.
The conversion constant-setting means has a target value-calculating means for determining the target value; and the target value-calculating means determines the target value depending on a hematocrit value when the examination site is brain of the specimen, and a region of interest for determining the conversion constant xcex3 is set to be a region including white matter of the brain. Accordingly, it is possible to correctly determine the conversion constant xcex3 for determining the cerebral blood flow rate.
In this arrangement, a relational expression for determining the arterial blood velocity constant from the expiration gas velocity constant on the basis of the conversion constant xcex3 is represented by:
Ka=xcex3xc3x97(1xe2x88x92exp(xe2x88x92Ke/xcex3))
wherein Ke represents the expiration gas velocity constant, and Ka represents the arterial blood velocity constant.
The xenon CT apparatus further comprises a display unit for displaying a distribution map of the blood flow rate and/or the conversion constant xcex3.
The above and other objects, features, and advantages of the present invention will become more apparent from the following description when taken in conjunction with the accompanying drawings in which a preferred embodiment of the present invention is shown by way of illustrative example.